1. Field of the Invention
The present invention relates to apparati and methods for controlling constituent mass densities within molecular beams, and more particularly to apparati and methods for rapidly varying the flux rate of epitaxial materials from effusion cells during the fabrication of semiconductor devices.
2. Description of Prior Art
Molecular beam epitaxy techniques which permit growing epitaxial layers have been described, and are well known in the art. In a typical molecular beam epitaxy (MBE) system, two or more source materials (generally a semiconductor material and at least one dopant material) are separately heated in effusion cells, thereby elevating their vapor pressures, to generate individual beams consisting of molecules (or atoms) of these materials. The individual beams of molecules then travel under molecular flow conditions toward the surface of a heated substrate where they react to deposit layers of predetermined composition on the substrate surface. The term "molecular flow" is herein used to refer to such flow conditions in which individual molecules move without undergoing direction altering collisions. Such conditions are most practically maintained in an evacuated environment, thus MBE is generally carried out in vacuum chambers.
Although widely used, the molecular beam epitaxy techniques have many inherent deficiencies which plague fabrication processes and limit production of compositionally graded structures. Compositionally graded structures are those in which the proportion of a given element or molecule varies with respect to the semiconductor matrix over the thickness of the structure. The production of such structures requires the capacity to vary the flux of a given molecular beam in a predetermined manner. In conventional MBE systems, flux rates of a given molecular (or atomic) species are determined by the temperatures, and associated vapor pressures, of the corresponding source materials in their effusion cells. In such conventional MBE systems, as described and illustrated in J. C. Anderson, et al., Materials Science 4th Ed., Chapman & Hall, N.Y., 1990, p. 464-6, selectively interposeable shuttering elements are positioned between beam sources and target substrates which, upon actuation into or out of the path of the molecular flow, affect the initiation or termination of epitaxial growth.
The fabrication of compositionally graded structures, as described above, requires a more graduated variation in the beam flux than may be provided by the conventional shuttering elements described in the prior art. While the obvious extension of the shuttering technique, which comprises partially inserting a shuttering element into the path of the molecular flow, would reduce the total flux of material, the resultant partial beam would not be uniform. This is so even when the effects of angular dispersion and spatial molecular averaging of the beam are taken into consideration. Non-uniformity in molecular beams of the sort described above cause lateral variation of molecular composition, not the depth variation which is desired.
Several techniques have been disclosed in the art which relates to methods and mechanisms for controllably varying beam fluxes. First is the method of adjusting the temperature, and thereby the vapor pressure, of the individual effusion cell. The levels of control, uniformity, and reproducibility of the beams and epitaxial layers generated by using this technique are limited. Significant time lags in adjusting the temperature of the source elements contribute to the uniformity problems as well as to a dramatic slowing of the production process. During temperature adjustment periods, the flow of source material must be interrupted by interposing shutter elements. During these interrupted periods of temperature adjustment, however, effusion from the source is not terminated. As a result of this, and because the shutter elements do not make a physical seal, a deposition of leaked material continues at an unregulated rate.
The second of the techniques for controllably varying beam fluxes includes the use of expensive needle valves, referred to, in combination with the effusion cells, as valved cracker cells. For example, U.S. Pat. No. 5,080,870 discloses an MBE system including a valve which controls molecular flux. In U.S. Pat. No. 5,080,870, and other valved cracker cell MBE systems, source material within an effusion cell is maintained at a constant temperature, thereby sustaining a constant vapor pressure within the cell. Opening the needle valve permits the comparatively higher pressure vapor within the cell to escape at a specific flux into the evacuated main MBE chamber. The resultant molecular beam is subsequently directed to a substrate. The extent to which the valve is opened determines the molecular flux of the beam.
Unlike conventional effusion cells, which produce beams of particles having an average energy directly proportional to the temperature of the source material, the energy and speed of the beam produced by a valved cracker cell is, in large part, determined by the difference in pressure across the valve. Forced expansion of the heated vapor through a valve causes it to cool. Valved cracker cells are, therefore, reasonably successful for use with materials which have relatively low boiling points. Arsenic is one such element, however, the majority of important materials (i.e. Ga, Al, In, Sb, Cd, Zn, Se, and Te) have higher boiling points. If these materials are used as sources within valved cracker cells, expansion of the heated vapor through the needle valves tends to cause condensation of the material within the valve. Over a short period of operation, the condensing vapor narrows the effective throat of the valve, thereby altering its performance. The build up of condensed material, in fact, may eventually clog the valve completely. These difficulties, which reduce the ability to effectively regulate of the flux of the beam, render valved cracker cells unsuitable for use in many MBE operations in the fabrication of advanced photonic and electronic devices with compositionally graded regions.
It is, therefore, an object of the present invention to provide a mechanism for use in combination with an effusion cell which affects rapid and controlled variation of the flux within a molecular beam which does not require adjustment of the temperature of the source material.
It is still another object of the present invention to provide a mechanism for rapidly varying the flux from an effusion cell which permits the use of a wide variety of important source materials.
It is still another object of the present invention to provide a mechanism for rapidly varying the flux of a molecular beam which does not use a valve and, therefore, does not become clogged by a condensing source vapor.
It is still another object of the present invention to provide a rapid flux varying effusion cell assembly which permits faster and less expensive MBE fabrication of compositionally graded devices.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.